RPM fluctuation at idle
Quote from tony-muscle on October 1, 2019, 10:23 pmQuote from Austin DeHaas on October 1, 2019, 9:36 pmExhaust leaks just past sensor can cause issues also.
I saw three small leaks within couple inches of the sensor. Luckily they are in a location that I can get to so I will weld those tomorrow night. I also have exhaust cut outs that don't seal perfectly. Can someone explain why a leak past the sensor can create issues? Are we talking about tiny leaks affecting or only large leaks?
Leaks don't always leak exhaust. Sometimes draw in air, thus increasing the oxygen in the stream. This can revert slightly and confuse the sensor. Wreaking havoc on the EFI.
So I was able to weld a couple holes I had downstream from the O2 sensor. This made no changes on this fluctuation. This is driving me crazy. I wonder if it has to do with the dizzy or CD ignition.
Here is graph showing my RPM fluctuation.
Help please!
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Quote from tony-muscle on September 30, 2019, 10:56 pmQuote from javelin_72 on September 30, 2019, 10:21 amI had a similar issue with a leaking header gasket which was only about 2" beyond the O2 sensor. Make sure your exhaust is not leaking. Just a thought...
Thanks. Today I checked under there and I had a couple of small leaks after the O2 sensor, but not before. When you mean "beyond", do you mean between the sensor and engine, or sensor and exhaust tip?
Sorry I didn't see this earlier, I meant between the O2 sensor and the exhaust exit. This really looks like a vacuum leak of some kind. Can you lock out the timing to make sure that's not causing the issue? Do you have timing going through the ECU? If not, what are you using for vacuum advance? I'd eliminate EVERYTHING and start from scratch: locked timing with no vacuum advance (or plug the vacuum advance if locking is too cumbersome). If that solves it, then the problem is probably in the timing. If not, then at least you have ruled that out. I just put on a 800Hp unit with timing control using the ECU to control the timing, and it is really running well right now without tinkering with it too much. I have another setup with a locked dist and it runs very smoothly as well. Don't give up.
Quote from javelin_72 on October 28, 2019, 9:47 pmQuote from tony-muscle on September 30, 2019, 10:56 pmQuote from javelin_72 on September 30, 2019, 10:21 amI had a similar issue with a leaking header gasket which was only about 2" beyond the O2 sensor. Make sure your exhaust is not leaking. Just a thought...
Thanks. Today I checked under there and I had a couple of small leaks after the O2 sensor, but not before. When you mean "beyond", do you mean between the sensor and engine, or sensor and exhaust tip?
Sorry I didn't see this earlier, I meant between the O2 sensor and the exhaust exit. This really looks like a vacuum leak of some kind. Can you lock out the timing to make sure that's not causing the issue? Do you have timing going through the ECU? If not, what are you using for vacuum advance? I'd eliminate EVERYTHING and start from scratch: locked timing with no vacuum advance (or plug the vacuum advance if locking is too cumbersome). If that solves it, then the problem is probably in the timing. If not, then at least you have ruled that out. I just put on a 800Hp unit with timing control using the ECU to control the timing, and it is really running well right now without tinkering with it too much. I have another setup with a locked dist and it runs very smoothly as well. Don't give up.
I don't have timing through the ECU yet. I have a Duraspark distributor powering a MSD StreetFire CD ignition. I check for vacuum leaks by removing every vacuum hose and by using the carb cleaner spray method. All seemed good.
For ignition, I had already tried with the vac advance unplugged with no difference. With the timing light I don't see much of an advance change when in idle, even when the RPMs go to 900+.
There have been troubles with Duraspark distributors. I have been told they do not produce a strong enough tach signal for the Fitech.
Quote from Austin DeHaas on October 31, 2019, 10:17 pmThere have been troubles with Duraspark distributors. I have been told they do not produce a strong enough tach signal for the Fitech.
Correct, but the distributor is activating the CDI module, and the module then activates the coil and send the RPM signal to the CPU. Since the coil and the EFI rpm signals come from the CDI, I think it shouldn't be an issue.
How much advance are you running at idle and is you vacuum canister hooked to a manifold or ported vacuum source? You mentioned that the timing didn't change when you reconnected vacuum to dizzy, so it sounds like ported. I would switch it to manifold go have more timing at idle which most engines really like. I know you checked for vacuum leaks, have you tried plugging the pcv valve? Depending on the cam, the valve could be opening and closing at idle; that's the reason for the push to use the new...and expensive...adjustable pcv valves.
Quote from Hugger1 on November 1, 2019, 6:56 pmHow much advance are you running at idle and is you vacuum canister hooked to a manifold or ported vacuum source? You mentioned that the timing didn't change when you reconnected vacuum to dizzy, so it sounds like ported. I would switch it to manifold go have more timing at idle which most engines really like. I know you checked for vacuum leaks, have you tried plugging the pcv valve? Depending on the cam, the valve could be opening and closing at idle; that's the reason for the push to use the new...and expensive...adjustable pcv valves.
I doubt it is related to the amount of advance. While tuning I have run between 10 and 14 with no difference to the idle fluctuation. I did try removing the PCV hose and plugged it with my finger. I did not see any difference.
My next step is to remove the distributor and CDI. I will install the old distributor retrofitted with Pertronix pickup and try it with and without the CDI. That is the last item in my check list.
Edit: I am using port vacuum.
Before removing the distributor you might want to try moving to manifold vacuum. Understand you've gone as high as 14 initial, but the vacuum advance will add another 12 to 16 on top of that. Won't cost anything to try and it might help smooth things out, especially if you have a decent cam in it.
I officially hit a dead end with this issue.
I replaced the Duraspark distributor with the old distributor using Pertronix. I tried this setup without the CD ignition. The issue at idle continued the same. The fluctuation did not change with more or less spark advance either. Something is happening that I am missing. The fluctuation is about 200 rpm, between 730 and 940 with a standard deviation of 40. Idle target is set to 810.